Mach Zehnder interferometers (MZIs) are used to modulate the amplitude or intensity of an optical signal, for instance in digital and analog communications and electric field sensors. Often, they are fabricated as monolithic devices on either Z-cut or X-cut lithium niobate or lithium tantalate.
A typical MZI includes two arms for dividing an optical input signal into two beams, which are then recombined. In one design, the arms have equal length. During operation, the relative phase of the two beams is controlled by some type of optical phase modulation in one or both arms in order to modulate the intensity of the output signal, which depends on the relative phase of the two beams.
In many applications, the drive voltage of the modulator is of particular interest as it affects link gain, sensor sensitivity, and drive power requirements for high speed, such as ˜40 Gigahertz (GHz) or faster, analog or digital links. Drive voltage sensitivity is characterized by the product between the drive voltage and the length of the device electrodes.
For devices of limited size, such as found in many optical communication applications, drive voltages tend to be relatively high, for example in the 4 to 5 Volt (V) range. On the other hand, low voltages often are desired in analog applications, where switching between the on and off states is characterized by Vπ, π being the total phase shift in the interferometer. As a result, such devices, when packaged, can be relatively long, for example in the 10 to 15 centimeter (cm) range. Generally, a Z-cut substrate is preferred when fabricating devices for which a low Vπ is important. Typical values for the voltage-length(VL) product in such devices are in the 10 to 12 Vcm range.
Since the voltage length product depends on the intrinsic electro-optic coefficient of the material used, considerable effort has been devoted to investigating materials with high electro-optical coefficients. For a given electro-optical material, the voltage-length product depends on the detailed geometric configuration of waveguides and electrodes. Mathematically, this factor can be described by an overlap integral between the electrical and optical fields. It can be shown that the overlap integral is strongly dependent on the exact geometry chosen.
Modulators based on a coplanar waveguide (CPW) electrode structure and having horn arrangement on lithium niobate (LiNbO3) substrates are disclosed in U.S. Pat. No. 6,304,685 B1, issued on Oct. 16, 2001 to W. K. Burns and U.S. Pat. No. 6,356,673 B1, issued on Mar. 12, 2002 to W. K. Burns, to the instant inventor, both applications being incorporated herein in their entirety by this reference. The use of ridge waveguides described in these patents was found to enhance the overlap integral, and to facilitate, simultaneously, velocity matching and near 50 Ohm line impedance.
In the prior CPW arrangement, a voltage is applied to a central hot electrode, which is disposed between two ground electrodes, with the vertical (Z-directed) electrical field component having opposite directions on the two arms of the interferometer. Thus, the phase changes accumulated in the two arms of the interferometer have opposite sign. Since the output of the interferometer depends on the difference of these phase changes, they add to produce the total phase change in the MZI. This is termed a “push-pull” configuration.
In the “push-pull” configuration associated with the CPW arrangement disclosed in U.S. Pat. No. 6,304,685 B1 and U.S. Pat. No. 6,356,673 B1, however, the electrical confinement is such that the overlap efficiency is uneven with respect to the two MZI waveguide arms. It is estimated that while the overlap integral for the waveguide under the central hot electrode contributes a large percentage of about 80% to the total phase change, the overlap integral for the MZI waveguide under the ground electrode contributes relatively little (about 20%) to the total phase changes.
Double coplanar strip (CPS) Mach Zehnder integrated electro-optical modulators are disclosed, for example, in US. Patent Application Publication No. 2003/0002766, by Pruneri et al., published on Jan. 2, 2003. In some of the arrangements shown in this reference, the substrate includes regions with mutually inverted ferroelectric domain orientations and the same drive voltage is applied to neighboring central electrodes. Electrical coupling between the neighboring central electrodes is prevented.
Domain reversal also has been proposed to make zero-chirp interferometers on Z-cut material.